This invention relates to a composition and the use there of for cleaning used supported silver catalysts as part of a process for regenerating such catalysts. Specifically, the invention relates to the removal of alkali metal-containing impurities from the used catalyst.
Supported sliver-based catalysts have been used industrially for many years for the oxidation of ethylene to ethylene oxide with oxygen or air. Most of the ethylene which is reacted is converted into ethylene oxide on the silver-impregnated catalyst support material and the remainder of the ethylene is converted almost exclusively to carbon dioxide and water. The goal is to react as much ethylene as possible, i.e. high productivity, such that the greater amount of the ethylene is converted to ethylene oxide, i.e. high selectivity.
It is known that the selectivity of these silver catalysts tends to decrease after the catalysts have been used for a number of years. I have found that one reason for the decrease in selectivity is the build-up of alkali metal-containing impurities on the catalysts. The decrease in selectivity results in less favorable economy of operation. It becomes advantageous to reactivate or regenerate the catalysts since an increase in selectivity of as little as one percentage point (selectivity equals 100 times the amount of ethylene converted to ethylene oxide divided by the total amount of ethylene consumed) can and will result in the savings of many thousands of dollars in a commercial operation.
There are several known methods for reactivating or regenerating silver catalysts. U.S. Pat. Nos. 4,051,068 issued Sept. 27, 1977, 4,123,385 issued Oct. 31, 1978, 4,125,480 issued Nov. 14, 1978, and 4,177,169 issued Dec. 4, 1979 disclose four such methods for regenerating silver catalysts. An essential part of the process of U.S. Pat. No. 4,125,480 is a washing step wherein the used catalyst is washed with one to ten times its volume of water or a mixture of water and an organic solvent before additional promoters are deposited on the catalyst. Treatment with water or water-containing solutions can be detrimental to catalyst performance. In addition, exposure of the reactors to aqueous solutions can result in corrosion of the reactor and the formation of iron oxides which can be detrimental to the ethylene oxide manufacturing process.
U.S. Pat. No. 4,186,106 issued Jan. 29, 1980 discloses a process to improve the activity of supported silver catalysts which comprises washing the catalyst with an inert organic liquid and then applying cesium, rubidium, or a mixture thereof to the catalyst. The use of such non-aqueous solvents may provide adequate cleaning of the catalyst and eliminate the problems involved with exposure of the catalyst and/or reactor to an aqueous environment. There is no mention of the deletorious effects of alkali metal-containing impurities on the catalyst. Furthermore, I have found that less washings (and/or washing time) are required to remove the same amount of alkali metal-containing impurities if a solubilizing agent is included in the inert organic liquid prior to the washing of the catalyst than if no solubilizing agent is used. Many inert organic liquids are ineffective in removing alkali metal contaminants by themselves, but become very effective with the inclusion of a solubilizing agent.